Print quality measuring method and print quality measuring apparatus

ABSTRACT

A representative color determining step is executed to determine, from image data, a representative color characterizing an image of a print, and positions of the representative color. A gray control color determining step is executed to determine, from the image data, a gray control color expressed in an achromatic color and positions of the gray control color. A reading step is executed to read an image of reference paper and an image of an actual print. Then, a calculating step is executed to carry out a comparative calculation of color data in the positions of the representative color of the image of the reference paper and color data in the positions of the representative color of the image of the print, and a comparative calculation of color data in the positions of the gray control color of the image of the reference paper and color data in the positions of the gray control color of the image of the print, to create control data for controlling ink feeding rates of a printing machine.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to print quality measuring methods andprint quality measuring apparatus.

[0003] 2. Description of the Related Art

[0004] In order to perform proper printing with a printing machine, itis necessary to control ink feeding rates properly. For controlling theink feeding rates, it has been conventional practice to measuredensities of control strips with a densitometer and determine fromdensity data whether the ink feeding rates are proper or not. However,the density data from the control strips alone is not necessarilysufficient for attaining a proper color tone and the like for a picturearea.

[0005] For this reason, a print quality measuring apparatus is usedwhich provides control data for controlling the ink feeding rates of aprinting machine. The control data is produced by comparing an image onreference paper and an image on an actual print.

[0006] The reference paper is also called proof paper, and serves as areference indicating a color tone of finished prints to obtain properprints. Printing paper actually printed is also called sampling paperwhich is extracted by the operator from a discharge station of aprinting machine at certain intervals during a printing operation. Theprinting is considered proper when the color tone on the sampling papersubstantially coincides with the color tone on the reference paper.

[0007] Where a print quality measuring apparatus is used for comparingan image on the reference paper and image on the actual print to producecontrol data for controlling the ink feeding rates of a printingmachine, it is difficult to attain an agreement in color tone and thelike in all areas between the image on the reference paper and image onthe actual print. For this reason, only certain areas of these imagesare compared. Such areas selected for comparison do not always assure aproper color tone and the like for areas characterizing the picture.

SUMMARY OF THE INVENTION

[0008] The object of this invention, therefore, is to provide a printquality measuring method and a print quality measuring apparatus forforming proper images in areas characterizing a picture, therebyobtaining proper prints from a printing machine.

[0009] The above object is fulfilled, according to this invention, by aprint quality measuring method for comparing an image of reference paperand an image of an actual print to create control data for controllingink feeding rates of a printing machine, the method comprising a readingstep for reading the image of the reference paper and the image of theactual print; a representative color determining step for determining,from image data, a representative color characterizing the image of theprint, and positions of the representative color; and a calculating stepfor carrying out a comparative calculation of color data in thepositions of the representative color of the image of the referencepaper and color data in the positions of the representative color of theimage of the print, to create the control data for controlling the inkfeeding rates of the printing machine.

[0010] With this print quality measuring method, proper images may beformed in areas characterizing a picture, thereby obtaining properprints from a printing machine.

[0011] In a preferred embodiment of the invention, the representativecolor and the positions thereof are determined for respective sectionscorresponding to ink keys in each ink well of the printing machine.

[0012] Preferably, the image data has three color components, therepresentative color determining step being executed to classify pixelsin each of the sections corresponding to ink keys, and determine therepresentative color and a position thereof from pixels included in apredetermined class interval.

[0013] The representative color determining step may be executed tocreate a histogram with tones of each of the three color components ofeach pixel in each of the sections, and select the representative colorand the position thereof from pixels included in a class interval ofmaximum frequency in the histogram.

[0014] In another embodiment, the print quality measuring method furthercomprises a gray control color determining step for determining, fromthe image data, a gray control color expressed in a substantiallyachromatic color and positions of the gray control color; wherein theoperating step is executed to create the control data for controllingthe ink feeding rates of the printing machine, by using results of acomparative calculation of color data in the positions of the graycontrol color of the image of the reference paper and color data in thepositions of the gray control color of the image of the print, as wellas results of the comparative calculation of the color data in thepositions of the representative color of the image of the referencepaper and the color data in the positions of the representative color ofthe image of the print.

[0015] Preferably, only the results of the comparative calculation ofthe color data in the positions of the gray control color of the imageof the reference paper and the color data in the positions of the graycontrol color of the image of the print are used when the representativecolor is devoid of one of the three color components.

[0016] In another aspect of the invention, a print quality measuringmethod is provided for comparing image data obtained by reading an imageof an actual print with one of platemaking data used at platemaking timeand image data created from the platemaking data, to create control datafor controlling ink feeding rates of a printing machine, the methodcomprising a reading step for reading the image of the actual print; arepresentative color determining step for determining a representativecolor characterizing the image of the print, and positions of therepresentative color, based on one of the platemaking data used atplatemaking time and the image data created from the platemaking data;and a calculating step for carrying out a comparative calculation ofcolor data in the positions of the representative color of the image ofthe print and the representative color, to create the control data forcontrolling the ink feeding rates of the printing machine.

[0017] In a further aspect of the invention, a print quality measuringapparatus is provided for comparing an image of reference paper and animage of an actual print to create control data for controlling inkfeeding rates of a printing machine, the apparatus comprising a readingunit for reading the image of the reference paper and the image of theactual print; a representative color determining unit for determining,from image data, a representative color characterizing the image of theprint, and positions of the representative color; and a calculating unitfor carrying out a comparative calculation of color data in thepositions of the representative color of the image of the referencepaper and color data in the positions of the representative color of theimage of the print, to create the control data for controlling the inkfeeding rates of the printing machine.

[0018] Other features and advantages of the invention will be apparentfrom the following detailed description of the embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] For the purpose of illustrating the invention, there are shown inthe drawings several forms which are presently preferred, it beingunderstood, however, that the invention is not limited to the precisearrangement and instrumentalities shown.

[0020]FIG. 1 is a perspective view of a print quality measuringapparatus according to this invention;

[0021]FIG. 2 is a side view of the print quality measuring apparatus;

[0022]FIG. 3 is a block diagram showing a principal structure of acontrol unit;

[0023]FIG. 4 is a flow chart of a print quality measuring operation;

[0024]FIG. 5 is a representative color determining operation;

[0025]FIG. 6 is an explanatory view showing positions of arepresentative color, a gray control color and a black control color;

[0026]FIG. 7 is an explanatory view schematically showing a readingstep;

[0027]FIG. 8 is a flow chart of a calculating sequence; and

[0028]FIG. 9 is a view showing data stored in a first and a second imagememories.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] An embodiment of this invention will be described hereinafterwith reference to the drawings. FIG. 1 is a perspective view of a printquality measuring apparatus according to the invention. FIG. 2 is a sideview of the apparatus. It is to be noted that light sources 13 and acontrol panel 15 are omitted from FIG. 2.

[0030] This print quality measuring apparatus includes a table 12disposed above a frame 11, a pair of light sources 13 arranged at rightand left sides of the table 12, an image pickup unit 14 disposed abovethe table 12, a control panel 15 disposed above one of the light sources13, an upper light-shielding plate 17 and a rear light-shielding plate18 supported by a pair of posts 16, an auxiliary light source 19attached to the rear light-shielding plate 18, and a control unit 20mounted inside the frame 11 for controlling the entire apparatus.

[0031] The table 12 is shaped planar for receiving a print thereon. Thetable 12 has a surface in the form of a suction plate for holding theprint by static electricity or vacuum suction. The surface of the table12 is inclined about 10 degrees for facility of operation by theoperator. The print held by suction on the inclined surface of the table12 is illuminated by the pair of light sources 13 arranged at theopposite sides.

[0032] The image pickup unit 14 disposed above the table 12 has adigital camera for separating, with a dichroic mirror, light emittedfrom the light sources 13 and reflected from the surface of the printinto the three primary color components of RGB, and receiving theindividual components with separate CCD arrays. With this image pickupunit 14, RGB data can be obtained from the print.

[0033] The control panel 15 is the touch panel type in the form of anLCD monitor having a pressure sensitive input function (also called atouch sensitive screen). This control panel 15 acts as both a displaydevice and an input device, and is connected to the control unit 20described hereinafter.

[0034]FIG. 3 is a block diagram showing a principal structure of thecontrol unit 20.

[0035] This control unit 20 includes a ROM 21 for storing operatingprograms necessary for controlling the apparatus, a RAM 22 fortemporarily storing data and the like during a control operation, a CPU23 for performing logic operations, and a first and a second imagememories 24 and 25. The control unit 20 is connected through aninterface 26 to the control panel 15, light sources 13 and image pickupunit 14 noted above. The control unit 20 is connected also to an imagedata source 27 storing image data to be printed, such as a hard disk oran image processing device. This control unit 20 acts as therepresentative color determining device, gray control color determiningdevice and calculating device of this invention.

[0036] Referring again to FIGS. 1 and 2, the upper light-shielding plate17 supported by the pair of posts 16 has a curved configurationextending in the fore and aft direction of the print quality measuringapparatus. The light-shielding plate 17 is installed in order tointercept light, such as light from indoor light sources, that wouldconstitute a regular reflection from the table 12. On the other hand,the rear light-shielding plate 18 supported between the pair of posts 16serves to intercept light coming from behind the print quality measuringapparatus.

[0037] The auxiliary light source 19 attached to the rearlight-shielding plate 18 serves to compensate for a lack of light on thetable 12 caused by the upper light-shielding plate 17 and rearlight-shielding plate 18. The auxiliary light source 19 is in the formof a fluorescent light or the like, which is turned off when reading aprint with the image pickup unit 14.

[0038] In the print quality measuring apparatus having the aboveconstruction, a representative color determining and other stepsdescribed in detail hereinafter are first executed by using image datastored in the image data source 27 of an image to be printed.

[0039] Next, reading steps are executed to read an image of referencepaper and an image actually printed. Specifically, reference paper isfirst placed on the table 12 and held thereon by suction. The referencepaper is illuminated by the light sources 13, and the image of thereference paper is read by the image pickup unit 14. Data of the imageof the reference paper is stored in the first image memory 24 of thecontrol unit 20. Next, a print extracted by the operator from adischarge station of a printing machine during a printing operation isplaced on the table 12, and held thereon by suction. This print isilluminated by the light sources 13, and the image of the print is readby the image pickup unit 14. Data of the image of the print is stored inthe second image memory 25 of the control unit 20.

[0040] Then, a calculating step to be described in detail hereinafter isexecuted to create control data for controlling ink feeding rates of theprinting machine. This control data is transmitted on-line or off-linethrough the interface 26 to the printing machine not shown.

[0041] Each step of the above print quality measuring operation will bedescribed next. FIG. 4 is a flow chart of the print quality measuringoperation.

[0042] When a print is measured in order to create the control data forcontrolling the ink feeding rates of the printing machine, arepresentative color characterizing the image to be printed andpositions of this color are first determined based on the data of theimage (step SI). This representative color determining step is executedby a subroutine shown in FIG. 5.

[0043] First, the image data of the image to be printed is fetched fromthe image data source 27 (step S11). This image data is platemaking dataof CMYK (cyan, magenta, yellow and black) for an image printed whenmaking printing plates, or image data created from this platemakingdata. This image data is supplied as PPF (Print Production Format) data,for example, according to the CIP3 (international Cooperation forIntegration of Prepress, Press and Postpress) standards. In thisspecification, black is referred to as K or Bk as appropriate.

[0044] Next, this PPF data (CMYK data) is converted to data with RGBtones (step S12). Considering that the CMYK data has 0 to 255 eight-bitvalues, the conversion is performed by the formulas (1)-(3) set outbelow. Negative RGB values are regarded as zero.

R=255−(C+K)  (1)

G=255−(M+K)  (2)

B=255−(Y+K)  (3)

[0045] Next, edge components are extracted and removed from the RGBimage (step S13). That is, when an edge amount which is a sum ofdifferences (absolute values) between a given pixel and pixels adjacentthereto in the four directions exceeds a fixed value, this given pixelis regarded as an edge pixel. Such edge pixels are excluded from thesubsequent process.

[0046] The RGB image is divided into sections corresponding to ink keysin each ink well of the printing machine (step S14). The subsequentsteps (step S15 et seq.) are executed for each divided section.

[0047] First, pixels with heavy contributions of Bk (black) ink areextracted and removed (step S15). That is, of the data of CMYK beforeconversion to RGB, pixels with a minimum value of CMY smaller than thevalue of K are regarded as pixels with heavy contributions of Bk, andare excluded from the subsequent process.

[0048] Next, a three-dimensional histogram is created with the RGBvalues of the remaining pixels (step S16). Specifically, athree-dimensional region is appropriately divided for every colorcomponent of RGB to form equally divided cubes with one side including apredetermined tonal range, and then a frequency distribution isdetermined for respective class intervals. A process of creating thisthree-dimensional histogram is described in detail in JapaneseUnexamined Patent Publication No. 11-296672 (1999) in the name ofAssignee herein.

[0049] In this embodiment, the histogram is created with the RGB values.Instead, a frequency distribution may be created with three componentsof a different color system, such as CMY values.

[0050] Next, a color expressed by the intervals of the histogram isweighted by multiplying the frequency by an appropriate factor (stepS17). When it is desired to give priority to flesh color in the pictureas representative color, its color gamut is multiplied by a largefactor. An interval having a maximum frequency after the multiplicationby the factor is determined to be the color gamut serving asrepresentative color, and only the pixels included in this interval areconsidered in the subsequent process.

[0051] In the above description, a histogram is created to determine acolor gamut serving as representative color. Alternatively, the operatormay designate a predetermined color gamut in place of the intervalhaving the maximum frequency. In this case, the calculation offrequencies is unnecessary.

[0052] Next, isolated points are removed from the pixels included in theinterval of maximum frequency resulting from the multiplication (stepS18). That is, outermost pixels of the areas formed by the pixelsincluded in the interval of maximum frequency resulting from themultiplication are removed as isolated points. This operation isrepeated until the total number of pixels in all areas becomes 1 or 0(step S19).

[0053] When the number of remaining pixels is 1, the CMYK values of thispixel is regarded as forming the representative color, and the positionof the pixel is regarded as the position of the representative color.When the number of remaining pixels is 0, the CMYK values of one of thepixels that remained to the last, e.g. a pixel near the center of thesections corresponding to the ink keys, are regarded as forming therepresentative color, and the position of this pixel is regarded as theposition of the representative color. In this way, a representativecolor and its position are determined (step S20).

[0054] The position of maximum area formed by the pixels (area of aseries of pixels) included in the interval of maximum frequency afterthe removal of isolated points is selected to be the position ofrepresentative color as noted above. This is done to avoid the influenceof errors caused by the intrinsic noise of the image pickup unit 14, andthe influence of alignment errors occurring when comparing color data ofa position of a representative color of the image on the reference paperdescribed hereinafter and color data of the position of therepresentative color of a printed image.

[0055] In the above embodiment, one pixel remaining after the removal ofisolated points is regarded as the pixel corresponding to therepresentative color, and the CMYK values of this pixel are regarded asforming the representative color. The representative color may beobtained from an average or weighted average of the CMYK values of thatpixel and a number of adjacent pixels. This measure is effective forlessening the influence of noise included in pixels.

[0056] Referring again to FIG. 4, a gray control color expressed in asubstantially achromatic color and its position are determined next(step S2).

[0057] This gray control color determining step is executed by asequence similar to that of the representative color determining stepshown in FIG. 5. However, in the gray control color determining step,step S17 in FIG. 5 is executed to multiply the intervals of thehistogram corresponding to gray by a factor of 1 or more, so that a grayportion is selected with priority in step S18. Of course, apredetermined color gamut may be designated directly as an intervalpresenting the gray control color. When the selected gray region failsto have a fixed area, that is when the number of repetitions made instep S19 does not reach a fixed number, the section corresponding to oneof the ink keys are regarded as being smaller than a predetermined grayarea, and no gray control color is determined.

[0058] Next, a Bk control color expressed in black and its positions aredetermined (step S3).

[0059] This Bk control color determining step is executed by a sequencesimilar to that of the representative color determining step shown inFIG. 5. However, in the gray control color determining step, step S15 inFIG. 5 is executed to extract and remove pixels with minor contributionsof Bk (black) ink. That is, of the CMYK data, the pixels with a maximumvalue of CMY larger than the value of K are regarded as pixels withminor contributions of Bk, and are excluded from the subsequent process.When the selected Bk region fails to have a fixed area, that is when thenumber of repetitions made in step S19 does not reach a fixed number,the section corresponding to one of the ink keys are regarded as beingsmaller than a predetermined Bk area, and no Bk control color isdetermined.

[0060]FIG. 6 shows explanatory views showing positions of therepresentative color, gray control color and Bk control color. In FIG. 6(a), D1-D7 denote positions of the representative color determined inthe representative color determining step (step S1). In FIG. 6 (b),G1-G7 denote positions of the gray control color determined in the graycontrol color determining step (step S2). In FIG. 6 (c), B2 and B6denote positions of the Bk control color obtained in the Bk colordetermining step (step S3).

[0061] The positions of the representative color, gray control color andBk control color are displayed, along with the image to be printed, onthe control panel 15 shown in FIG. 1. The operator may confirm thepositions of the gray control color and Bk control color displayed onthe control panel 15, and may, as necessary, change the positions of therepresentative color as shown in FIG. 6 (d).

[0062] In the embodiment shown in FIG. 6, the image is divided intoseven sections corresponding to the ink keys in each ink well of theprinting machine. Further, in this embodiment, Bk control points arepresent only in the second section from the right and in the secondsection from the left.

[0063] Referring again to FIG. 4, the image of the reference paper andthe image of an actual print are read (steps S4 and S5).

[0064] Specifically, as shown in FIG. 7, the image of reference paper S1is first read by the image pickup unit 14. The RGB values and positionsof the representative color, the RGB values and positions of the graycontrol color and the RGB values and positions of the Bk control colorare stored in the first image memory 24 shown in FIG. 3. At this time,as shown in FIG. 7, an image 100 acquired from the image data and animage 101 read by the image pickup unit 14 are different from each otherin the number of the pixels, resolution, margin and so on. It istherefore necessary to correlate the two images in order to obtaincorrectly the RGB values and positions of the representative color, theRGB values and positions of the gray control color and the RGB valuesand positions of the Bk control color. In this case, the images may bedisplayed on the control panel 15 for the operator to work with theimages manually or carry out a pattern matching. An example of patternmatching techniques is disclosed in Japanese Patent Application No.2002-205117 in the name of Assignee herein.

[0065] Next, as shown in FIG. 7, the image of actual print S2 is read bythe image pickup unit 14. As in the case of the reference paper notedabove, positions corresponding to the positions of the representativecolor, gray control color and Bk control color derived from the PPF dataare determined. The RGB values and positions of the representativecolor, the RGB values and positions of the gray control color and theRGB values and positions of the Bk control color are stored in thesecond image memory 25 shown in FIG. 3. In this case also, as shown inFIG. 7, an image 100 acquired from the image data and an image 101 readby the image pickup unit 14 are different from each other in the numberof the pixels, resolution, margin and so on. Thus, the images may bedisplayed on the control panel 15 for the operator to work with theimages manually or carry out a pattern matching.

[0066] Referring again to FIG. 4, after the image of the reference paperand the image of the actual print are read, a comparative calculation iscarried out on the data obtained by reading the image of the referencepaper and the image of the actual print, to create control data forcontrolling the ink feeding rates of the printing machine (step S6).This calculating step is executed by a subroutine shown in FIG. 8.

[0067] As a result of the steps of reading the image of the referencepaper and the image of the actual print (steps S4 and S5), the firstimage memory 24 stores the data of the reference paper as partiallyshown in FIG. 9 (a), and the second image memory 25 stores the data ofthe print as partially shown in FIG. 9 (b). As shown in FIG. 6 (c), noBk control color is selected for the first section. Thus, no data ispresent in the corresponding columns.

[0068] The CMYK values shown in FIGS. 9 (a) and 9 (b) are the PPF datafetched in the image data fetching step (step S11) shown in FIG. 5,which data are common to the reference paper and the print. On the otherhand, the RGB values are the data obtained by reading the image of thereference paper and the image of the actual print. When a differenceexceeding a fixed difference exists between the two groups of RGBvalues, proper prints are not being produced.

[0069] For this reason, a density difference is first calculated foreach of the representative color, gray control color and Bk controlcolor (step S61). Before this calculation, the RGB values shown in FIG.9 are converted to CMYK values. This step is taken because the inkfeeding rates are controlled more suitably by the CMYK values which areindexes linked to variations in ink quantity than by the RGB valuesrepresenting color.

[0070] Next, the density differences of CMYK calculated are multipliedby factors selected according to dot percentages (step S62). That is, alarger density difference occurs in an area of large dot percentage thanin an area of small dot percentage. The multiplication is carried out inorder to secure exact information on the calculated density differencesirrespective of dot percentage. At this time, the dot percentages arecalculated by using the CMYK values shown in FIG. 9. However, the dotpercentages may by calculated by using the RGB values shown in FIG. 9.

[0071] The above multiplication provides control data for controllingthe ink feeding rates. At this time, the control data for controllingthe feeding rate of black ink may be obtained by multiplying the densitydifference of Bk control color by a factor selected according to the Kvalue. On the other hand, the control data for controlling the feedingrate of each of CMY inks is created by using one or both of a valueobtained by multiplying the density difference of the representativecolor by a factor selected according to the C, M or Y value, and a valueobtained by multiplying the density difference of the gray control colorby a factor selected according to the C, M or Y value (step S63).

[0072] That is, when the representative color does not include one ofCMY, only the value obtained by multiplying the density difference ofthe gray control color by a factor selected according to the C, M or Yvalue is used for the color not included or for all of CMY. In othercases, control data may be created by using only the value obtained bymultiplying the density difference of the representative color by afactor selected according to the C, M or Y value, or by using acompromise in an appropriate ratio of the value obtained by multiplyingthe density difference of the representative color by a factor selectedaccording to the C, M or Y value, and the value obtained by multiplyingthe density difference of the gray control color by a factor selectedaccording to the C, M or Y value.

[0073] After obtaining the control data for controlling the feedingrates of CMY inks and the control data for controlling the feeding rateof K ink in the above steps, these data are transmitted to the controlunit of the printing machine (step S64) to end the entire process.

[0074] In the embodiment described above, a representative color and thelike are determined for each of the sections corresponding to the inkkeys in each ink well of the printing machine. Instead, a representativecolor and the like of an entire image may be determined first, andpriority may be given to a color close to the above color to determine arepresentative color and the like for each section.

[0075] In the above embodiment, a representative color and its positionsare determined by using platemaking data or image data based on theplatemaking data (PPF data). Instead, a representative color and itspositions may be determined based on image data obtained by reading thereference paper. Further, instead of the image data obtained by readingthe reference paper, platemaking data used for preparing the referencepaper or image data based on the platemaking data may be used. In thiscase, this invention may be implemented also where image data such asPPF data is unavailable as where, for example, a platemaking operationis not digitized.

[0076] This invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

[0077] This application claims priority benefit under 35 U.S.C. Section119 of Japanese Patent Application No. 2002-210872 filed in the JapanesePatent Office on Jul. 19, 2002, the entire disclosure of which isincorporated herein by reference.

What is claimed is:
 1. A print quality measuring method for comparing animage of reference paper and an image of an actual print to createcontrol data for controlling ink feeding rates of a printing machine,said method comprising: a reading step for reading the image of thereference paper and the image of the actual print; a representativecolor determining step for determining, from image data, arepresentative color characterizing the image of the print, andpositions of the representative color; and a calculating step forcarrying out a comparative calculation of color data in the positions ofsaid representative color of the image of said reference paper and colordata in the positions of said representative color of the image of saidprint, to create the control data for controlling the ink feeding ratesof the printing machine.
 2. A print quality measuring method as definedin claim 1, wherein said representative color and the positions thereofare determined for respective sections corresponding to ink keys in eachink well of the printing machine.
 3. A print quality measuring method asdefined in claim 2, wherein said image data has three color components,said representative color determining step being executed to classifypixels in each of said sections corresponding to ink keys according totones of each of the three color components, and determine saidrepresentative color and a position thereof from pixels included in apredetermined class interval.
 4. A print quality measuring method asdefined in claim 3, wherein said representative color determining stepis executed to create a histogram with the tones of each of the threecolor components of each pixel in each of said sections, and select saidrepresentative color and the position thereof from pixels included in aclass interval of maximum frequency in said histogram.
 5. A printquality measuring method as defined in claim 4, wherein the position ofthe representative color selected is a position having a maximum areaformed by the pixels included in said class interval.
 6. A print qualitymeasuring method as defined in claim 5, wherein said image data fordetermining said representative color is one of platemaking data used atplatemaking time, image data obtained by processing the platemakingdata, and image data obtained by reading said reference paper.
 7. Aprint quality measuring method as defined in claim 1, furthercomprising: a gray control color determining step for determining, fromthe image data, a gray control color expressed in a substantiallyachromatic color and positions of the gray control color; wherein saidoperating step is executed to create the control data for controllingthe ink feeding rates of the printing machine, by using results of acomparative calculation of color data in the positions of said graycontrol color of the image of said reference paper and color data in thepositions of said gray control color of the image of said print, as wellas results of the comparative calculation of the color data in thepositions of said representative color of the image of said referencepaper and the color data in the positions of said representative colorof the image of said print.
 8. A print quality measuring method asdefined in claim 7, wherein only the results of the comparativecalculation of the color data in the positions of said gray controlcolor of the image of said reference paper and the color data in thepositions of said gray control color of the image of said print are usedwhen said representative color is devoid of one of said three colorcomponents.
 9. A print quality measuring method as defined in claim 7,wherein the control data for controlling the ink feeding rates of theprinting machine is created by selectively using the results of thecomparative calculation of the color data in the positions of saidrepresentative color of the image of said reference paper and the colordata in the positions of said representative color of the image of saidprint, and the results of the comparative calculation of the color datain the positions of said gray control color of the image of saidreference paper and the color data in the positions of said gray controlcolor of the image of said print, or by using a compromise in anappropriate ratio of the results of the two comparative calculations.10. A print quality measuring method for comparing image data obtainedby reading an image of an actual print with one of platemaking data usedat platemaking time and image data created from the platemaking data, tocreate control data for controlling ink feeding rates of a printingmachine, said method comprising: a reading step for reading the image ofthe actual print; a representative color determining step fordetermining a representative color characterizing the image of theprint, and positions of the representative color, based on one of saidplatemaking data used at platemaking time and said image data createdfrom the platemaking data; and a calculating step for carrying out acomparative calculation of color data in the positions of saidrepresentative color of the image of said print and said representativecolor, to create the control data for controlling the ink feeding ratesof the printing machine.
 11. A print quality measuring method as definedin claim 10, wherein said representative color and the positions thereofare determined for respective sections corresponding to ink keys in eachink well of the printing machine.
 12. A print quality measuring methodas defined in claim 11, wherein each of said platemaking data used atplatemaking time and said image data created from the platemaking datahas three color components, said representative color determining stepbeing executed to classify pixels in each of said sections correspondingto ink keys, and determine said representative color and a positionthereof from pixels included in a predetermined class interval.
 13. Aprint quality measuring method as defined in claim 12, wherein saidrepresentative color determining step is executed to create a histogramwith tones of each of the three color components of each pixel in eachof said sections, and select said representative color and the positionthereof from pixels included in a class interval of maximum frequency insaid histogram.
 14. A print quality measuring method as defined in claim13, wherein the position of the representative color selected is aposition having a maximum area formed by the pixels included in saidclass interval.
 15. A print quality measuring method as defined in claim10, further comprising: a gray control color determining step fordetermining a gray control color expressed in a substantially achromaticcolor and positions of the gray control color, based on one of saidplatemaking data used at platemaking time and said image data createdfrom the platemaking data; wherein said operating step is executed tocreate the control data for controlling the ink feeding rates of theprinting machine, by using results of a comparative calculation of colordata in the positions of said gray control color of image data obtainedby reading the image of said print and said gray control color, as wellas results of the comparative calculation of the color data in thepositions of said representative color of the image of said print andsaid representative color.
 16. A print quality measuring method asdefined in claim 15, wherein only the results of the comparativecalculation of the color data in the positions of said gray controlcolor of the image data obtained by reading the image of said print andsaid gray control color are used when said representative color isdevoid of one of said three color components.
 17. A print qualitymeasuring method as defined in claim 15, wherein the control data forcontrolling the ink feeding rates of the printing machine is created byselectively using the results of the comparative calculation of thecolor data in the positions of said representative color of the imagedata obtained by reading the image of said print and said representativecolor, and the results of the comparative calculation of the color datain the positions of said gray control color of the image data obtainedby reading the image of said print and said gray control color, or byusing a compromise in an appropriate ratio of the results of the twocomparative calculations.
 18. A print quality measuring apparatus forcomparing an image of reference paper and an image of an actual print tocreate control data for controlling ink feeding rates of a printingmachine, said apparatus comprising: reading means for reading the imageof the reference paper and the image of the actual print; representativecolor determining means for determining, from image data, arepresentative color characterizing the image of the print, andpositions of the representative color; and calculating means forcarrying out a comparative calculation of color data in the positions ofsaid representative color of the image of said reference paper and colordata in the positions of said representative color of the image of saidprint, to create the control data for controlling the ink feeding ratesof the printing machine.
 19. A print quality measuring apparatus asdefined in claim 18, wherein said representative color and the positionsthereof are determined for respective sections corresponding to ink keysin each ink well of the printing machine.
 20. A print quality measuringapparatus as defined in claim 19, wherein said image data has threecolor components, said representative color determining means beingarranged to classify pixels in each of said sections corresponding toink keys, and determine said representative color and a position thereoffrom pixels included in a predetermined class interval.
 21. A printquality measuring apparatus as defined in claim 20, wherein saidrepresentative color determining means is arranged to create a histogramwith tones of each of the three color components of each pixel in eachof said sections, and select said representative color and the positionthereof from pixels included in a class interval of maximum frequency insaid histogram.
 22. A print quality measuring apparatus as defined inclaim 21, wherein the position of the representative color selected is aposition having a maximum area formed by the pixels included in saidclass interval.
 23. A print quality measuring apparatus as defined inclaim 22, wherein said image data for determining said representativecolor is one of platemaking data used at platemaking time, image dataobtained by processing the platemaking data, and image data obtained byreading said reference paper.
 24. A print quality measuring apparatus asdefined in claim 18, further comprising: gray control color determiningmeans for determining, from the image data, a gray control colorexpressed in a substantially achromatic color and positions of the graycontrol color; wherein said operating means is arranged to create thecontrol data for controlling the ink feeding rates of the printingmachine, by using results of a comparative calculation of color data inthe positions of said gray control color of the image of said referencepaper and color data in the positions of said gray control color of theimage of said print, as well as results of the comparative calculationof the color data in the positions of said representative color of theimage of said reference paper and the color data in the positions ofsaid representative color of the image of said print.
 25. A printquality measuring apparatus as defined in claim 24, wherein only theresults of the comparative calculation of the color data in thepositions of said gray control color of the image of said referencepaper and the color data in the positions of said gray control color ofthe image of said print are used when said representative color isdevoid of one of said three color components.
 26. A print qualitymeasuring apparatus as defined in claim 24, wherein the control data forcontrolling the ink feeding rates of the printing machine is created byselectively using the results of the comparative calculation of thecolor data in the positions of said representative color of the image ofsaid reference paper and the color data in the positions of saidrepresentative color of the image of said print, and the results of thecomparative calculation of the color data in the positions of said graycontrol color of the image of said reference paper and the color data inthe positions of said gray control color of the image of said print, orby using a compromise in an appropriate ratio of the results of the twocomparative calculations.
 27. A print quality measuring apparatus forcomparing image data obtained by reading an image of an actual printwith one of platemaking data used at platemaking time and image datacreated from the platemaking data, to create control data forcontrolling ink feeding rates of a printing machine, said apparatuscomprising: reading means for reading the image of the actual print;representative color determining means for determining a representativecolor characterizing the image of the print, and positions of therepresentative color, based on one of said platemaking data used atplatemaking time and said image data created from the platemaking data;and calculating means for carrying out a comparative calculation ofcolor data in the positions of said representative color of the image ofsaid print and said representative color, to create the control data forcontrolling the ink feeding rates of the printing machine.
 28. A printquality measuring apparatus as defined in claim 27, wherein saidrepresentative color and the positions thereof are determined forrespective sections corresponding to ink keys in each ink well of theprinting machine.
 29. A print quality measuring apparatus as defined inclaim 28, wherein each of said platemaking data used at platemaking timeand said image data created from the platemaking data has three colorcomponents, said representative color determining means being arrangedto classify pixels in each of said sections corresponding to ink keys,and determine said representative color and a position thereof frompixels included in a predetermined class interval.
 30. A print qualitymeasuring apparatus as defined in claim 29, wherein said representativecolor determining means is arranged to create a histogram with tones ofeach of the three color components of each pixel in each of saidsections, and select said representative color and the position thereoffrom pixels included in a class interval of maximum frequency in saidhistogram.
 31. A print quality measuring apparatus as defined in claim30, wherein the position of the representative color selected is aposition having a maximum area formed by the pixels included in saidclass interval.
 32. A print quality measuring apparatus as defined inclaim 27, further comprising: gray control color determining means fordetermining a gray control color expressed in a substantially achromaticcolor and positions of the gray control color, based on one of saidplatemaking data used at platemaking time and said image data createdfrom the platemaking data; wherein said operating means is arranged tocreate the control data for controlling the ink feeding rates of theprinting machine, by using results of a comparative calculation of colordata in the positions of said gray control color of image data obtainedby reading the image of said print and said gray control color, as wellas results of the comparative calculation of the color data in thepositions of said representative color of the image of said print andsaid representative color.
 33. A print quality measuring apparatus asdefined in claim 32, wherein only the results of the comparativecalculation of the color data in the positions of said gray controlcolor of the image data obtained by reading the image of said print andsaid gray control color are used when said representative color isdevoid of one of said three color components.
 34. A print qualitymeasuring apparatus as defined in claim 32, wherein the control data forcontrolling the ink feeding rates of the printing machine is created byselectively using the results of the comparative calculation of thecolor data in the positions of said representative color of the imagedata obtained by reading the image of said print and said representativecolor, and the results of the comparative calculation of the color datain the positions of said gray control color of the image data obtainedby reading the image of said print and said gray control color, or byusing a compromise in an appropriate ratio of the results of the twocomparative calculations.